The present invention relates to a wave shaping circuit and more particularly to a circuit for converting an analog waveform to a digital waveform.
Although, the inventive circuit is of general utility wherever analog to digital wave shaping is required, a particular application is in conjunction with optical scanning apparatus utilized to decode bar codes. In particular, such scanning apparatus is used in point of sale (POS) applications where the bar codes appear on articles, either printed directly thereon or on associated indentification tags or labels. One particular form of bar code now widely utilized in the grocery industry is the Universal Product Code (UPC) which consists of a symbol having a plurality of parallel dark and light (or black and white) bars.
As the scanning apparatus scans the symbol, a waveform is generated having an amplitude which varies as a function of time from positive peaks associated with the sensed light bars to negative peaks associated with the sensed dark bars (or vice versa depending on the polarity of the light sensing system employed).
In practical applications, the analog signal contains wide variations in amplitude, depending on such factors as the printing colors used for the light and dark bars, reflectivity of the printing medium, presence of debris over the symbol, noise created by ambient light pickup, and so forth. Such variations may exist from symbol to symbol or, within a particular symbol, from bar to bar. However, decoding circuitry cannot function properly with analog signals which have such characteristics.